Measurement of the Z Resonance Parameters at LEP

The properties of the Z resonance are measured from the analysis of 4.5 million Z decays into fermion pairs collected with the \Aleph\ detector at L EP. The data are consistent with lepton universality. The resonance parameters are measured to be $\MZ=(91.1885 \pm 0.0031)~\Gevcc$, $\GZ= (2.4951 \pm 0.0043)~\GeV$, $\spol=(41.559 \pm 0.058)$~nb and, combining the three lepton flavours $\Rl= 20.725\pm 0.039$. The corresponding number of light neutrino species is $N_{\nu}=2.983\pm0.013$ and the strong coupling constant is $\alpha_s(\MZ) = 0.114\pm {0.004} \pm 0.002_{\rm{QCD}} +\,0.005\,\log_{10}\left[\frac{\MH}{150\,{\rm GeV}/c^2 }\right]\,$. The lepton pair forward-backward asymmetry is measured to be $\Afbpol=0.0173 \pm 0.0016$ from which the effective weak mixing angle is derived: $\swsqeffl = 0.23089\pm 0.00089\,$. The measurement of the leptonic width $\Gll=84.02\pm0.15$~MeV leads to a determination of the effective $\rho$ parameter $\rho_{\rm eff}^{\rm lept} = 1.0064 \pm 0.0018$. The data support the Standard Model and favour a light Higgs.